Since a single crystal sapphire is a material having advantages such as resistance to breakage, high strength and optical transmittance of wide wavelength range, it is used in a variety of fields such as logistics, aviation, optics, medical field and the like. Particularly, demands for sapphire ingots are exponentially growing recently for manufacture of light emitting diode (LED) products.
FIG. 1 is a view showing an example of a device for growing a single crystal in a conventional sapphire ingot growing furnace.
Describing the process of manufacturing an ingot, first, after raw materials are put into a crucible 2 of the growing furnace, a sapphire material is melted by operating a heating means.
Then, after a seed (seed crystal) 3 is mounted at the lower end of an ingot rod, the seed 3 is approached to the center of the surface of the melt inside the crucible 2 to contact with or partially dip into the melt by the operation of a seed operating means 4.
Growth of a sapphire single crystal begins by adjusting temperature of the crucible 2 in such a state, and then, a single crystal ingot of a shape close to a cylinder can be obtained by properly adjusting the pulling speed, position and temperature.
In the initial stage of the process, the seed 3 slowly ascends after contacting with the melt 1 by the operation of the seed operating means 4, and the point and time of the seed contacting with the melt 1 depend on visual observation and empirical judgment of a user.
FIG. 2 is a view schematically showing a flow state of the surface of a melt inside a growing furnace.
Referring to FIG. 2, an island is a portion indicated by reference numeral 6 in FIG. 2, and when melting of the melt is in a favorable normal state by heating, the island 6 appears as a distinguishable region having an area of a coin size. The flowing surface of the melt 1 is separated into an ascending portion 7 of high temperature, ascending from the bottom to the surface, and a descending portion 8 of slightly low temperature compared with the ascending portion 7, where the melt ascended to the surface descends again. A flow boundary line 9 appears around the descending portion 8 extended from the center in a radial shape as the melt in the neighborhood is gathered and descends, and such a flow boundary line 9 is created around the island 6 in a radial shape.
The island 6 is a portion where the melt descends, which is a region where the temperature is low compared with surrounding areas where the melt ascends. Accordingly, growth of a single crystal begins from the island 6 by contacting the seed 3 with the island 6 having a temperature lower than that of the surrounding areas.
The island 6 is not always generated at the center of the surface of the melt, and its position is changed according to the flow of the melt.
Accordingly, since a handler should handle descending of the seed 3 at a proper time point when the island 6 enters the descending position of the seed 3, there is a difficulty in that inside of the growing furnace of high temperature should be observed frequently with naked eyes to catch the time point.
Furthermore, since the seed 3 should accurately contact with the island 6, the position of which continuously changes, to produce a single crystal ingot of high quality, the work needs to be done by a highly skilled handler.